That guy, is the person who enters an online video game just to mess with people. The "That Guy" is in every game room. He/She can range from someone who just messes around killing his teammates or someone with just a high pitch voice.

That Guy: Oops I killed my whole team again.
You: Oh great its That Guy.

Kid with high pitch voice: Hey guys are you ready for fun?!
You: We have officially found That Guy.

The one person who, with a broken-up couple in a crowded room, says something loudly that would imply that the couple is dating.
This inevitably happens for each badly ended relationship once word spreads to everyone except "that guy" that they are through.